Non-linear end-of-line clipper circuit for pulsers



May 30, 1967 w. SMITH v 33 9 NON-LINEAR END-OF-LINE CLIPPER CIRCUIT FORPULSERS Filed June 9, 1964 INVENTOR. WILL/AM SMITH vATTORNEY UnitedStates Patent Oflice 3,322,975 Patented May 30, 1967 sion Filed June 9,1964, Ser. No. 373,892 4 Claims. (Cl. 307-93) The present inventionrelates generally to anon-linear end-of-line clipper circuit forprotecting a line type pulser against load arcs which are commonlyexperienced when pulsing loads such as klystrons or magnetrons.

It is well known in the design of line-type pulser circuits commonlyutilized in pulsing are susceptible loads such as klystrons ormagnetrons that faults can and readily do occur in the load. Forexample, some reverse voltage is required on a pulse-forming network,following the pulse, to shut olf the gas or solid state type switch tubewhich is normally used to trigger the pulse into the load, inpreparation for the next pulse-forming network cycle. However, if theload arcs or otherwise faults the pulse forming network isover-discharged, causing its voltage to nearly fully reverse. This fullreverse voltage appearing at the anode of the switch tube of the pulserimmediately after a conductive cycle is quite often destructive to theswitch tube. In addition, if this full inverse voltage is removed, itshould be done in as short a time as possible as compared with thehalf-cycle of oscillation of the charging reactor and the capacitance ofthe pulserforming'network, or the pulse-forming network will seriouslyovercharge. Overcharging of the pulse-forming network causes excessiveload voltage and promotes repetitious arcing of the load with cumulativevoltage buildup on the pulseforming network in successive chargingcycles until eventual circuit failure occurs.

There are prior art circuits available which utilize a combination of ashunt-diode and a series resistor connected thereto, to drain thepulse-forming network and thus remove the reverse voltage on thepulse-forming network due to load arcing. In circuits heretoforedesigned with relatively slow acting shunt diode circuitry, the chargingreactor is still overstressed for a significant period of time followingload arcing. Furthermore, full inverse voltage still appears at theanode of the switch tube promptly following each load arc with probableshortening of the life of such a switch tube.

An additional prior art circuit with improved protection for pulsercircuitry consists of a very low impedance diode and series resistormatching the pulse-forming network and connected at the end thereofremote from the switch tube. With such an end-of-line clipper circuit aload arc at the switch tube end causes the pulse-forming network energyto be delivered to the clipper resistor without appearance of highinverse voltage at the switch tube end. The charging inductor is notoverstressed. If the switch tube shuts oil, the next charging cycle isentirely normal. In the past, gas diodes have been utilized in thecircuit as the very low impedance diode, but inherent delay in achievingrequisite ionization has tempered their eifectiveness in short timepulse work. Pretriggered thyratrons have also been utilized in thecircuit, but require complicated and expensive associated circuitry inorder to operate successfully.

The recent availability of reliable low-impedance silicon diodeassemblies have made this type of end-of-line clipper advantageous.However, performance is hampered because some inverse voltage isnecessary on the pulse-forming network following a normal pulse as wellas following a pulsed load arc in order to shut off the switch tube. Amatched end-of-line clipper causes serious deionization or switchshut-off problems by too promptly removing or preventing the appearanceof inverse voltage on the pulse-forming network. This is particularlytrue when utilizing inductive deionization control.

The presentinvention overcomes the above-mentioned shortcomings ofvarious prior art protective circuits by providing an end-of-lineclipper circuit capable of preventing the appearance of high inversevoltage at the anode of the switch tube therein following a load arc,while allowing stable deionization of the switch tube even under loadarc and other drastic fault conditions.

Accordingly, it is an object of the present invention to provide anend-of-line clipper circuit for use with linetype pulsers wherein theclipper circuit utilizes a serially connected non-linear resistor.

It is another object of the present invention to provide a non-linearend-of-line clipper circuit capable of preventing the appearance ofundesired high inverse voltage at the anode of the switch tube in aline-type pulser following a load arc.

Still another object of the present invention is to provide a non-linearend-of-line clipper circuit capable of rapidly draining the storedenergy from the pulse-forming network of a line-type pulser except forthat energy necessary to cause switch tube deionization.

Yet another object of the present invention is to provide a non-linearend-of-line clipper circuit capable of protecting a line-type pulseragainst load arcs while further preventing overstressing of the charginginductor following load arcing.

Other objects and advantages will be apparent in the followingdescription and claims considered together with the accompanyingdrawing, in which;

FIGURE 1 is a schematic diagram exemplifying the mechanism of thepresent invention, as utilized in conjunction with a conventionalline-type pulser circuit; and

FIGURE 2 is a schematic diagram depicting, in detail, the circuit ofFIGURE 1.

Referring more particularly to FIG. 1, there is shown a non-linearend-of-line clipper circuit 10 in accordance with the invention, asutilized in a conventional line-type pulser herein exemplified as acharging inductor 12, a pulse-forming network (PFN) 14 and gas switchtube 16. The PFN 14 is coupled to a load such as for example, a klystronor magnetron (not shown) by means of an output terminal 18 with theother terminal of the load being connected to ground. The PFN 14includes a tapped inductive element with capacitors having one sideconnected at top, input and output terminals of said inductive elementand the other side connected to a common point. The PFN 14 is chargedthrough the reactor 12 by a suitable D.C. power supply (not shown) whichis connected to one end of the inductive element of the pulser by meansof an input terminal 2% At such time as the PFN 14 is charged,triggering of the gas switch tube 16 provides a closed current paththrough groundby which the PFN 14 is discharged into the loa V Theend-of-line clipper circuit 10 in accordance with the inventioncomprises linear resistor means 22, nonlinear thyrite resistor means 24and silicon diode assembly means 26, serially connected together. Thefree end of linear resistor means 22 such as a common fixed resistor isconnected to one side, i.e., the second end of the inductive element ofthe pulse-forming network 14, and the thereof opposite the sideconnected to the inductive por- I tion. The clipper circuit 10 assemblyis therefore connected in parallel across the last capacitive element ofPFN 14. The positive or anode electrode of the silicon diode assemblymeans 26 is connected to the PFN 14, andthe negative or cathodeelectrode is connected to the nonlinear thyrite resistor means 24.

In operation, for very high pulse currents which are generated by loadarcing, the thyrite resistor means 24 has a value less than theimpedance of the pulse-forming network 14. Thus the resistor means 22 iseffectively inserted to supplement the resistor means 24, to provideaccurate matching. However, when the circuit elements are chosen todeliberately leave very low inverse voltages on the pulse-formingnetwork 14 to cause shut-off of the gas switch tube 16, the resistanceof the thyrite resistor means 24 is much higher than the impedance ofthe pulse-forming network 14. Hence the inverse voltage present remainson the pulse-forming network a sufficient period to enable the gasswitch tube 16 to cease conducting.

If the total resistance of thyrite resistor means 24 and resistor means2 exceeds the impedance of the PFN 14 by an order of magnitude, there isthe possibility that switch 16 will are back due to inverse voltage, andthe .behavior of the PFN 14 energy may cause failure of the silicondiode assembly means 26 due to reversal of its voltage immediatelyfollowing conduction. Accordingly, an optimum safe design for theend-of-line clipper circuit elements is satisfactorily chosen where thesum of the resistances of the thyrite resistor 24 and the resistor 22means under load are conditions is close to the impedance of thepulse-forming network 14 at the lowest voltage operating level, andwhere the resistance of the thyrite resistor means 24 is much greaterthan the impedance of the pulse-forming network 14 for the level ofinverse voltage necessary to deionize the gas tube 16. Such action issatisfactorily achieved by choosing a value for resistor means 22 of theorder of one-half the pulseforming network impedance, and by adjustingthe resistor means 24 to a value of the order of one-half thepulseforming network impedance, at the lowest load are current level.

Referring to FIG. 2 there is shown, in greater detail, the circuit ofFIG. 1, including examples of the specific values and types of elementshereinbefore generally described. More particularly, linear resistormeans 22 comprises for example a plurality of ohm, 200 watt resistors 28disposed two in series, with four of such serially connected resistorsconnected in parallel. The non-linear thyrite resistor means 24 isformed of, for example, 24 six inch diameter thyrite resistors 30 of thesilicon carbide type such as General Electrics No. 69W60100, wherein theresistors 30 are disposed threein series, with eight of such seriallyconnected resistors connected in parallel. The thy-rite resistors arechosen from the wide range of such resistors available, i.e., having athyrite current characteristic of the form i=kv where i is the currentthrough the resistor, v is the applied Voltage, k is the conductivity ofthe unit in amperes at 1 volt applied, and n is a constant depending onthe resistor material composition. Values of n thy-rite resistorsapplicable with the invention may be chosen within the range of from 3to 8. It is to be understood that although the invention is hereindescribed utilizing thyrite resistors as the non-linear resistor means24, there are other devices available which may be substituted therefor,e.g. Western Electric Companys silicon carbide varistors, Globar TypeBNR voltage sensitive resistors, etc. The silicon diode assembly means26 is formed of serially connected diodes 32, for example of the type1N1l96A, wherein each of the diodes has connected thereacross, inparallel electrical relation therewith, a 510,000 ohm 1 watt resistor34, and a 0.01 microfarad 1 kilovolt capacitor 36. Each diode 32 isshunted with a resistor 34 and a capacitor 36 to insure equal voltagedivision for each of the serially connected plurality of diodes 32. Thecharging voltage applied to thepulse forming networks 14 is of the orderof 40 to 42 kilovolts for the circuit parameters of above mention.

While the invention has been disclosed with respect to a preferredembodiment, it will be apparent to those skilled in the art thatnumerous variations and modifications may be made within the spirit andscope of the invention and it is not intended to limit the inventionexcept as defined in the following claims.

What is claimed is:

1. A non-linear end-of-line clipper circuit for protecting against loadarcs a line-type pulser having a pulseforming network comprising:resistor means coupled at one end thereofto the output end of one sideof said pulseforming network; said resistor means including a linearresistor serially connected to a variable non-linear thyrite resistor;low impedance diode assembly means including an anode and a cathodeelectrode, wherein said cathode is connected to the second end of saidresistor means, and said anode is connected to the output end of theother side of said pulse-forming network, whereby said resistor meansand said diode means are serially connected together in parallelrelation across said pulse-forming network. i

2. The non-linear endof-line clipper circuit in accordance with claim 1wherein said diode means comprises a silicon diode assembly ofrelatively low impedance.

3. The non-linear end-of-line clipper circuit in accordance with claim 2wherein the resistances of said linear resistor and of said thyriteresistor are each made equal to approximately one-half the impedance ofthe pulseforming network at the lowest load arc current level.

4. The non-linear end-of-line clipper circuit in accordance with claim 2wherein said silicon diode assembly further comprises a silicon diode, aresistor connected in parallel relation across said silicon diode, and acapacitor connected in parallel relation across said silicon diode.

References Cited UNITED STATES PATENTS 2,438,962 4/1948 Burlingame 32892,815,445 12/1957 YUCht 3=1591 X 2,967,275 1/1961 Brown 328--9 X3,063,012 1/1962 Jackson 328---9 X ORIS L. RADER, Primary Examiner.

T, J. MADDEN, Assistant Examiner,

1. AN NON-LINEAR END-OF-LINE CLIPPER CIRCUIT FOR PROTECTING AGAINST LOADARCS A LINE-TYPE PULSER HAVING A PULSEFORMING NETWORK COMPRISING:RESISTOR MEANS COUPLED AT ONE END THEREOF TO THE OUTPUT END OF ONE SIDEOF SAID PULSEFORMING NETWORK; SAID RESISTOR MEANS INCLUDING A LINEARRESISTOR SERIALLY CONNECTED TO A VARIABLE NON-LINEAR THYRITE RESISTOR;LOW IMPEDANCE DIODE ASSEMBLY MEANS INCLUDING AN ANODE AND A CATHODEELECTRODE, WHEREIN SAID CATHODE IS CONNECTED TO THE SECOND END OF SAIDRESISTOR MEANS, AND SAID ANODE IS CONNECTED TO THE OUTPUT END OF THEOTHER SIDE OF SAID PULSE-FORMING NETWORK, WHEREBY SAID RESISTOR